This application is concerned with a timer circuit for controlling the start motor of a diesel engine.
Diesel engines are generally provided with an electric start motor which cranks the engine at a speed of the order of 100 RPM. The drive train which connects the engine with a transmission or other load often has a torsional resonant speed or speeds and the drive train is designed to place these resonances below the idling speed of the engine. Typically, a resonant speed may occur between the cranking speed and the low idle speed (e.g., 600 RPM) as at about 200 RPM. When a cold engine is started, it is common to operate the start motor for a sufficient length of time to ensure acceleration past the low speed resonance. However, when a hot engine is restarted after a short shut-down, it will sometimes sustain fire at a speed as low as 100 RPM. The engine at this speed has very little torque, insufficient to accelerate through resonance. If the operator turns off the start motor at such a low speed, damage to the drive train may result. If, however, the start motor remains energized, the added torque it provides ensures that the engine will accelerate through the drive train resonance. We have found that with a typical diesel engine, the start motor should be operated for at least 0.8 second.
A principal feature of the invention is a diesel engine start motor control circuit which includes an operator actuated start switch, a timer initiated by actuation of the start switch and having an output with a minimum duration which is independent of the time for which the start switch is actuated, and a start motor circuit energized by the timer output.
Another feature is that the start motor circuit is energized in response to a combination of the timer output and actuation of the start switch so that the start motor circuit remains energized for the longer of the duration of the timer output and start switch actuation. More particularly, the timer has a monostable flip-flop circuit which is actuated by the start switch and has an output with a duration which is independent of the time for which the start switch is actuated. A bistable flip-flop latch circuit has an input responsive to a combination of the timer output and actuation of the start switch and has an output which controls energization of the start motor circuit. The bistable flip-flop latch circuit has a reset input which is responsive to the concurrent absence of actuation of the start switch and the timed output from the timer so that the bistable latch circuit is not reset until the timer is timed out and the operator has released the start switch.
A further feature of the invention is the provision of safety features in the start motor control circuit. One such safety circuit provides for resetting the monostable timer flip-flop when the vehicle disconnect switch is closed. Another compares the condition of the start motor circuit with the inputs from the start switch and timer. If they are not in agreement, the circuit is disabled by blowing a fuse.